1. (Field of the Invention)
The present invention relates to a film comprising a polymer capable of forming an optically anisotropic melt phase, a laminate comprising a base material applied with the polymer film, a method of making them and a multi-layered parts-mounted circuit board utilizing the laminate.
In the description that follows, the polymer capable of forming an optically anisotropic melt phase is referred to as the "liquid crystal polymer".
2. (Description of the Prior Art)
Recently in the field of electric and electronic industry, demands for flexible printed circuit boards (hereinafter referred to as "FPC boards") are increasing to cope with the need to make electric and electronic appliances compact and lightweight. The standard method of making the FPC boards comprises preparing a laminate made up of a base film having at least one of its opposite surfaces applied with a metal foil such as, for example, a copper foil and forming a pattern of electric circuits on the laminate. In most cases, a film of polyethylene terephthalate is generally used for the base film. However, since the polyethylene terephthalate film has an insufficient heat resistance, it has been found problematic in that when electric and/or electronic component parts are mounted on the FPC board and when the FPC board with the electric and/or electronic component parts thereon is subsequently dipped in a solder bath, the base film in the FPC board is susceptible to bulging, peel-off and/or thermal deformation. In view of this, the film made of the liquid crystal polymer excellent in heat resistance has now gained attentions as a material for the base film.
However, since the liquid crystal polymer generally has a high heat resistance, and when the liquid crystal polymer is desired to be molded into a film, not only is a relatively high molding temperature required accompanied by consumption of a relatively large amount of energies, but also the liquid crystal polymer itself tends to be thermally decomposed. Although of the various liquid crystal polymers the liquid crystal polymer capable of being molded at a relatively low molding temperature is available, it has often been found that the resultant film made of that liquid crystal polymer tends to have a low heat resistance and can, therefore, not be suited as a material for the heat resistant base film. Accordingly, a method has been suggested in, for example, the Japanese Laid-open Patent Publication No. 3-152132 wherein, after a film has been prepared from the liquid crystal polymer having a low molding temperature, the resultant liquid crystal polymer film is heat-treated at a temperature not higher than the melting point Tm of the liquid crystal polymer, but not lower than 200.degree. C. under the atmosphere of vacuum or reduced pressure to thereby impart a heat resistance to the liquid crystal polymer film.
The prior art method such as discussed above requires the heat treatment to be carried out for a substantial length of time under the vacuum or reduced pressure. Also, no means is employed to allow the liquid crystal polymer film to retain its shape and the film tends to loose its shape when heated to a temperature approaching the melting point of the liquid crystal polymer and, therefore, the film having an excellent appearance is hardly obtainable. More specifically, if the film itself is heat-treated at a temperature not lower than the thermal deformation temperature at which thermal deformation is initiated, the film is susceptible to deformation and stress build-up and thermal deformation is pronounced particularly where the film has a small thickness. To avoid this thermal deformation, the film is required to be heat-treated at a temperature region not higher than the thermal deformation temperature at which the thermal deformation starts. This heat-treatment requires an increased length of time until the film gains a required heat resistance, accompanied by reduction in productivity. On the other hand, if the heat-treatment is carried out with the h eating temperature increased to a value in the vicinity of the melting point Tm of the film, the productivity may be improved. However, the method has not yet been made available that is effective to accomplish the intended heat treatment optimally without allowing the film to be thermally deformed and to retain its shape throughout the heat-treatment process. In addition, in order for the film to be available as a commercial and/or industrial product, the film must be of a kind capable of being manufactured consistently and continuously, however, the method accomplishing this has not yet been known in the art.
According to the Japanese Patent Publication No. 55-20008, the heat-treatment of liquid crystal polymer filaments has been suggested in which the liquid crystal polymer filaments are heat-treated at a temperature lower by about is 20.degree. C. than the flow initiation temperature thereof to increase the strength by 50%.
The Japanese Laid-open Patent Publication No. 2-133347 discloses a method wherein the liquid crystal polymer filaments are heat-treated to a temperature ranging from the melting point of aromatic polyester, the material for the liquid crystal polymer filaments, to a temperature lower by 50.degree. C. than such melting point to thereby provide filaments of a high strength having a high Young's modulus of elasticity. However, the heat treatment of the liquid crystal polymer filaments disclosed therein is quite different from the heat treatment, such as carried out in the practice of the present invention as will be described later, in which the film to be heat-treated is kept flat, which heat treatment may be termed "planar heat treatment".
The Japanese Laid-open Patent Publication No. 8-90570 discloses a method of heat-treating a liquid crystal polymer film, which comprises subjecting a liquid crystal polymer film in contact with a film support (a base material) to a fusion heat-treatment, cooling the both to form a layered structure comprising the solidified liquid crystal polymer layer and the film support, again subjecting the layered structure to a heat treatment to allow the solidified liquid crystal polymer layer to be heated at a temperature ranging from about 150.degree. C. to a temperature lower by 30.degree. C. than the thermal deformation temperature at which the liquid crystal polymer layer may start its thermal deformation, and then separating the liquid crystal polymer layer from the film support. However, according to this known method, the heat-treatment in which the liquid crystal polymer film is melted is carried out at a temperature not lower than the melting point of the liquid crystal polymer film (In this respect, the heat treatment employed in the practice of the present invention is different from this method.) and the heat-treatment subsequent to the solidification is carried out at a temperature lower by 30.degree. C. than the thermal deformation temperature (Contrary to this, the heat treatment employed in the present invention is carried out at a temperature higher than this thermal deformation temperature). Accordingly, this known method is incapable of providing the liquid crystal polymer film having a high heat resistance and a high strength.